Electrical overstress of electronic components due to electrostatic discharge (ESD) is a serious problem in the electronics industry. The damaging effects of electrostatic discharge on integrated circuits are well known, and it is generally recognized that such damage can occur at any point during the manufacture, testing, repair, and even normal use of electronic equipment using such integrated circuits. Despite precautions which have been adopted in the industry, the problem has become more severe, due in part to the more widespread use of integrated circuits, the smaller sizes of devices and higher packaging densities, and due in part to the fact that newer generations of integrated circuits have greater susceptibility to electrical overstress at lower voltages than earlier generations.
The cost to industry due to electrostatic discharge is very high, and is measured in terms of the cost of the destroyed circuits, the time spent in troubleshooting and replacing damaged circuits, and in terms of economic loss and disruption caused by failures of equipment in the field. This is due to the fact that while some electrostatic discharge causes immediate failure of circuits which would be detected by functional tests in the manufacturing process, in other instances the electrostatic discharge causes latent damage which may cause a circuit to fail at some subsequent time. Such failures while equipment is in the field are even more costly because of the down time for equipment, for example, expensive computer systems, and because of the greater time and expense of repairing equipment in the field as compared to in the factory. Further, for some types of critical electronic equipment, for example, in the fields of medicine or aviation, such failures could pose safety hazards.
Typical static control measures used in the electronics industry include wrist straps for personnel which are connected to ground for removing static charges, special antistatic clothing for personnel, special containers for holding circuit boards or circuits, special temperature and humidity controls for work areas, conductive flooring and static dissipative shoes. These techniques are all useful in controlling the static buildup problem, but problems still occur.
It is generally thought that the personnel wrist strap grounding system is the most effective single measure for static control. The wrist strap consists of an elastic cuff having conductive material woven therein which fits snugly around the wrist of a person involved in assembly, repair or other handling of the integrated circuits or circuit boards. The wrist strap connects through a lead wire to a plug which is intended to be plugged into a grounding jack. Numerous grounding jacks are provided around the work benches and other work areas involved. The purpose is to provide a path to bleed off or discharge any static level which would be built up on the person's skin, and thus prevent a static voltage buildup which could damage the circuits. A resistor is provided in the lead wire for safety purposes to limit current through the body in case the person should inadvertently come in contact with an electrical power source. The resistance is high enough to substantially limit any such accidental current to safe levels, and is low enough to keep the discharge time for static charges on the body fast enough to avoid damage to the circuits. Typically, a one-megohm resistor is used, although in some cases a lower resistance of 500 kilohms might be used for certain newer types of integrated circuits having lower electrical overstress voltage limits.
Wrist straps may fail to provide adequate static charge dissipation under some circumstances. The resistance of the skin can vary from individual to individual, and can be affected by factors such as perspiration or lack thereof, room temperature and humidity. In addition, it has been found that a leading cause of wrist strap grounding system failure is due to broken lead wires. The lead wire connecting from the wrist strap and terminating in the grounding plug is subject to a considerable amount of physical movement while being worn during a typical work day. The person will typically plug into and unplug from grounding jacks in the work area dozens of times each day. In addition, the person's hand may be in continual motion while working. All of these factors place great strain on lead wires, with the result that the wires or the resistors within the lead wires will break with a surprising degree of frequency. Since a failure of the wrist strap system creates a potential for electrostatic discharge damage until the defective system is found and replaced, periodic testing of wrist strap effectiveness is to be recommended, and numerous test devices are available on the market. Typically they are ohmmetertype devices wherein the person wearing the wrist strap plugs the grounding lead wire into the test device then touches a conductor with the opposite hand. The ohmmeter device then measures resistance through the complete circuit including the skin to wrist strap contact, the lead wire and its resistor. The device then indicates that the system is satisfactory; that the resistance too low, indicating a shorted resistor and a potential personal hazard; or that the resistance is too high or open circuit, indicating that the system will not protect against static.
These types of testers generally work well when used frequently and regularly, but even then problems will occur. Even if wrist strap systems are tested daily, when a failure does occur, and it can be assured that failures eventually will occur, they will go undetected throughout the rest of the day, during which time the person wearing the defective wrist strap system may have handled hundreds of circuits, circuit board assemblies or the like. The standard industry approach to this problem has been to rely on a secondary static discharge path through the worker's footwear to conductive flooring material installed in the electronic assembly areas. However, experience has shown that this reliance is in many cases misplaced.
In the course of development of this invention, it was found that resistance levels from a person through his footwear to conductive flooring is not only higher than the resistance through a proper wrist strap system, but also that it is subject to a much greater degree of variability than for wrist straps. The composition of the sole of the shoe, the type of material in the stockings, and individual variability in skin resistance and foot perspiration, in addition to the usual temperature and humidity factors, cause great variability. Leather soled shoes may have low conductivity if there is high humidity or if the leather otherwise has a certain moisture content, but they have very high resistance when completely dry. Special ESD shoes have been developed with a composition sole having a known degree of conductivity, but still the other factors mentioned above cause such variability in resistance that it has been found that at a given time a large percentage of the personnel working in a static protected zone may have inadequate grounding through their footwear. For example, it has been found that a draft along the floor in an area of a room can slightly chill the feet of persons working in that area, which can lead to a decrease in foot perspiration and an increase in resistance.
Because of these problems, a program of checking the static discharge path through the shoe should also be implemented in addition to checking wrist strap systems. The test should be quick and simple to perform, because otherwise there may be a tendency to skip the test. However, no effective or reliable footwear conductivity path measurement apparatus is available.